1. Field of the Invention
This invention relates generally to a system and method for extending the life of the electrode catalyst in a fuel cell stack and, more particularly, to a system and method for extending the life of the electrode catalyst in a fuel cell stack by maintaining the stack potential above a certain value during a system stand-by mode to prevent or limit stack voltage cycling.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte there between. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode reactant input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant input gas that flows into the anode side of the stack.
A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It has been discovered that a typical fuel cell stack will have a voltage loss or degradation over the lifetime of the stack. It is believed that the fuel cell stack degradation is, among others, a result of voltage cycling of the stack. Voltage cycling occurs when the platinum catalyst particles used to enhance the electro-chemical reaction transition between an oxidized state and a non-oxidized state, which causes dissolution of the particles. When the platinum particles transition between the non-oxidized or metal state and an oxidized state, oxidized ions in the platinum are able to move from the surface of the MEA towards the membrane and probably into the membrane. When the particles convert back to the metal state, they are not in a position to assist in the electro-chemical reaction, reducing the active catalyst surface and resulting in the voltage degradation of the stack.
Oxidation of platinum particles in a fuel cell as a result of voltage cycling creates a passivation layer in the cell electrode that prevents the particles from going into solution and being absorbed into the membrane. In other words, oxidation of the platinum particles in a fuel cell reduces the possibility of a reduction in catalyst surface area, which reduces cell degradation. Although the discussion herein refers to the catalyst as being platinum, those skilled in the art will readily understand that other metals can be used as a catalyst and that the catalyst may be in various concentrations, particle sizes, support material, etc.
If the voltage of the fuel cell stack is less than about 0.9 volts, the platinum particles are not oxidized and remain a metal. When the voltage of the fuel cell stack goes above about 0.9 volts, the platinum crystals begin to oxidize. A low load on the stack may cause the voltage output of the fuel cell stack to go above 0.9 volts. The 0.9 volts corresponds to a current density of about 0.2 A/cm2, depending on the power density of the MEA, where a current density above this value does not change the platinum oxidation state. The oxidation voltage threshold may be different for different stacks and different catalysts.
Many factors influence the relative loss in surface area of the platinum particles relating to voltage cycling, including peak stack voltage, temperature, stack humidification, voltage cycling dynamics, etc. Lower stack voltage set-points offer greater protection against degradation, but higher stack voltage set-points provide increased system efficiency. Thus, the control for various fuel cell systems often requires the stack to at least operate at a minimum power level so that, in at least one case, the cell voltages are prevented from rising too high because frequent voltage cycles to high voltage can cause a reduction in the active platinum surface area of the cathode and anode electrodes, as discussed above.
Typically, in known fuel cell systems, a fixed voltage limit is used to set the stack minimum power level to prevent unwanted voltage cycling. For example, a typical voltage suppression strategy may use a fixed voltage set-point, such as 850-900 mV, and prevent the stack voltage from rising above that value. If the fuel cell power controller is not requesting power, or is requesting minimal power, the power generated by the stack necessary to maintain the cell voltage levels at or below the fixed voltage set-point is provided to certain sources where the energy is stored or dissipated. For example, the excess power may be used to charge a high voltage battery in a fuel cell system vehicle.
U.S. Patent Application Publication No. US 2006/0147770 A1, published Jul. 6, 2006, titled, Reduction of Voltage Loss Caused by Voltage Cycling by Use of A Rechargeable Electric Storage Device, assigned to the assignee of this application and herein incorporated by reference, discloses a fuel cell system that charges a vehicle battery in order to maintain the cell voltage below a predetermined fixed voltage set-point.
When a fuel cell system on a vehicle is in an idle mode, such as when the vehicle is stopped at a stop light, where the fuel cell stack is not generating power to operate system devices, air and hydrogen are generally still being provided to the fuel cell stack, and the stack is generating output power. This power is typically used to recharge the battery until an upper state of charge (SOC) limit of the battery is reached, where if the battery is charged beyond this upper limit, the battery may be damaged. When this SOC limit is reached, the battery load on the stack is removed, which increases the stack voltage, but causes voltage cycling referred to above that decrease the life of the stack. If the fuel cell system is turned off during the idle condition, then the problem of providing a load on the stack when the battery has reached its maximum SOC does not need to be addressed. Also, providing hydrogen to the fuel cell stack when it is in the idle mode is generally wasteful because operating the stack under this condition is not producing very much useful work, if any.
For these and other fuel cell system operating conditions, it may be desirable to put the system in a stand-by mode where the system is consuming little or no power, the quantity of hydrogen fuel being used is minimal and the system can quickly recover from the stand-by mode so as to increase system efficiency and reduce system degradation. U.S. patent application Ser. No. 12/723,261, titled, Standby Mode for Optimization of Efficiency and Durability of a Fuel Cell Vehicle Application, filed Mar. 12, 2010, assigned to the assignee of this application and herein incorporated by reference, discloses one process for putting a fuel cell system on a vehicle in a stand-by mode to conserve fuel.
When a fuel cell stack goes into the stand-by mode and is turned off, the voltage on the stack drops to zero, and when the stand-by mode is over and the stack is restarted, the voltage on the stack is increased. Thus, the above-described voltage cycling occurs that will reduce the performance of the catalyst based on the number of times the stand-by mode is entered and ended. It is possible to limit the load on the stack in the early time that the stack is in the stand-by to limit the voltage cycling. However, additional steps can be taken to limit loss of catalyst.